1. Technical Field
The present disclosure relates to MRI and, more specifically, to a method for fat fraction quantification in MRI.
2. Discussion of Related Art
Quantifying a level of fat and water within a particular region of the body has important diagnostic value. For example, quantification of fat and water may be helpful in diagnosing and monitoring the progression of fatty liver disease. Additionally, the ability to accurately quantify fat within various regions of the body may be used to measure certain risks posed by the presence of visceral fat.
The Dixon technique, first described by W. T. Dixon in 1984, exploits a chemical shift of approximately 3.4 ppm between fat and water. This chemical shift results in a shift in the resonance frequencies of fat and water. When used in conjunction with a number of image acquisition schemes such as two-dimensional RF spin echo, two-dimensional turbo spin echo, two-dimensional and three-dimensional gradient echo, spiral, RARE, GESFIDE, and others, this difference in resonance frequencies can be exploited to produce fat-only and water-only images of the body. Such images may be produced by comparing a sample image taken when the fat and water signals are exactly in-phase with each other (a 0 degree phase shift) with a sample image taken when the fat and water signals are exactly out of phase (a 180 degree phase shift).
For example, where the in-phase echo image is represented as I1 and the opposed phase echo image is represented as I2, the water image W and the fat image F may be computed as:
                    W        =                              1            2                    ⁢                      (                                          I                1                            +                              I                2                                      )                                              (                  1          ⁢          a                )                                F        =                              1            2                    ⁢                      (                                          I                1                            -                              I                2                                      )                                              (                  1          ⁢          b                )            
However, as this approach for distinguishing between fat and water relies on the phase of the resonance signals, calculating the water image and the fat image are highly susceptible to phase errors such as those resulting from B0 inhomogeneity (φ) and phase errors resulting from other system imperfections (φ0) such as eddy current, concomitant gradients, B1 inhomogeneity and the like. Thus the effects of phase errors in the in-phase and opposed-phase echo images I1 and I2 may be approximated according to the following:I1=(W+F)eiφ0  (2a)I2=(W+F)ei(φ0+φ)  (2b)
Moreover, in certain cases, T1 weighting and T2* signal loss may interfere with accurate fat/water separation. Accordingly, known approaches to quantification of fat and water ratios employ a two-dimensional imaging strategy such as GRE and use interleaved multi-slice, multi-echo imaging with longer repetition times (TR). Using these known two-dimensional approaches, T2* effects can be compensated and T1 weighting effects can be reduced, but spatial resolution is sacrificed. Conversely, known three-dimensional approaches improve the spatial resolution but commonly use a two echo acquisition strategy with no consideration for T1 weighting and T2* decay when generating separate fat and water images. Thus, existing approaches for quantification of fat and water ratios suffer from various limitations associated with reduced spatial resolution, long repetition times, T1 weighting, T2* decay contamination and/or long acquisition times.